In theory, these make sense and reduce anti-consumer inefficiency like scalping that individual venues are not equipped to deal with. In practice, they extract fees. It's not that they are bad, per se, just that if the opportunity exists, someone with a spreadsheet will spot it, likely with the best intentions but no eye to the overall impact.

EDIT: To be clear, I think these services are a net good. Stubhub allows me to get sports tickets at a reduced price if someone can't go to the game. Ticketmaster stops people from spamming the system to gobble up tickets, it's just that these industries are now going to want a fee for that and we end up back where we started. I'm sure at one time brokers were helpful as well (a landlord free way to compare properties).